Display screen or a portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or a portion thereof with a graphical user interface showing a new design; and,

FIG. 2 is a front view of the design of FIG. 1 applied to a smartphone environment.

The broken lines showing a smartphone (in FIG. 2), the broken lines showing an image taken by a camera (within a rectangle in FIGS. 1 and 2), and the other broken lines form no part of the claimed design. 

CLAIM The ornamental design for a display screen or a portion thereof with a graphical user interface, as shown and described. 